Lockable container for transporting and for storing semiconductor wafers
BACKGROUND OF THE INVENTION 
     The present invention relates to a container for transporting and for storing semiconductor wafers situated in processing cassettes. More particularly, the present invention relates to containers comprising a bottom plate, a hood and means for non-positive locking of the hood and the bottom plate. 
     In semiconductor manufacture, semiconductor wafers are manufactured and further-processed under clean room conditions. Containers whose interior offers a clean room climate are therefore necessary for storing the semiconductor wafers and for transporting the semiconductor wafers to the various processing locations. To that end, the containers should be optimally tight and should not release any contaminating substances, for example exhalations of material, themselves. 
     Standardization rules for transport containers as recited in the publication of Document No. 1332 of Semiconductor Equipment and Materials Institute dated 10 Jan. 1986 serve the purpose of defining containers that are interchangeable, that comprise standardized dimensions for robot manipulation and that are suitable for the acceptance of standardized processing cassettes. 
     This publication also provides recommendations concerning the demands that the component parts of a transport container should meet. The recommended standardizations are summarized under the term &#34;Standard Mechanical Interface for Wafer Cassette Transfer&#34; standard (SMIF Standard). 
     A lockable container for transporting and for storing semiconductor wafers situated in processing cassettes comprising a bottom plate, a hood and at least two locking levers for non-positive locking of the hood and the bottom plate that meets the SMIF Standard is disclosed by U.S. Pat. No. 4,582,219. This container comprises a rigid semiconductor wafer holder secured to the inside wall of the hood, this holder touching the semiconductor wafers situated in the processing cassette when the container is in its closed condition. Given a closed container, further, the semiconductor wafer holder is also held by a lock situated at the bottom plate. One disadvantage of this container is that it comprises component parts that are fashioned such that they cause friction when opening and closing the container and during transport, whereby contaminating particles can be released. Moreover, the container does not comprise any seal element between bottom plate and hood that ensures the particle tightness of the container. When closing the container, the projecting edge of the bottom plate is merely pressed against the edge of the hood by the spring power of the locking levers. 
     European Patent Application 87117836.4 also discloses a transport container for transporting and for storing semiconductor wafers situated in a semiconductor wafer cassette. Although this transport container can be closed in a dust-tight fashion and comprises a friction-free semiconductor wafer holder, it has an involved, complex design comprising an interchangeable, bipartite inside container. 
     Transport containers for semiconductor wafers that are particle-tight, contamination-free, meet the SMIF Standard and are distinguished by a simple structure at the same time are not previously known. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a container for transporting and for storing semiconductor wafers situated in a processing cassette that protects the semiconductor wafers against contamination due to the environment, does not release any particles during loading and unloading that contaminate the semiconductor wafers and that comprises a simple structure. The container should also be capable of being designed according to the rules of the SMIF Standard. 
     In a container of the type initially described, this object is achieved in that 
     (a) a seal element for air-tight closing of the container is provided between a hood and a bottom plate; and 
     (b) a semiconductor wafer securing means, movable by film hinges and secured to the inside wall of the hood is provided; this securing means is moved into securing position by contact with the bottom plate when the container is closed. 
     The developments and improvements include but are not limited to: 
     (a) A seal element between the hood and the bottom plate comprising a surface sealing lip attached to the bottom plate, and an opposing sealing surface integral to the hood. 
     (b) A seal element between the hood and the bottom plate comprising an elastically deformable sealing ring covered by a film strip; film strip being not-tensed and acting to raise the pressure in the container when the container is closed. 
     (c) A semiconductor wafer securing means comprising an attachment plate, a securing plate and a guide plate. These plates movably connected by film hinges to each other and the attachment plate connected to the inside surface of the hood. The guide plate contacts the bottom plate upon closure of the container. 
     Further developments and improvements of the invention are set forth in the following description provided below with reference to exemplary embodiments. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Eight figures shall be considered with reference to the exemplary embodiments set forth below, these figures being executed as schematic illustrations. 
     FIG. 1 shows a closed container in a perspective view; 
     FIG. 2 shows the hood of the container of FIG. 1 and a bottom plate as sectional views generally along the line II--II entered in FIG. 1; 
     FIG. 3 shows a locking lever for closing the hood and the bottom plate of FIG. 2 in a perspective view; 
     FIG. 4 shows a portion of a section of the closed container of FIG. 1 comprising the bottom plate of FIG. 2 generally along the section line IV--IV of FIG. 1; 
     FIG. 5 shows the semiconductor wafer securing means of FIG. 2 in a perspective view; 
     FIG. 6 shows a section through the seal element shown in FIG. 2 and shows a sealing lip when the container is nearly closed; 
     FIG. 7 shows a section through a seal element comprising a sealing lip in a different embodiment of the bottom plate; 
     FIG. 8 shows a section through a seal element comprising an elastically deformable seal ring when the container is nearly closed; and 
     FIG. 9 shows the seal element of FIG. 8 when the container is closed. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an embodiment of a container incorporating the principles of the present invention. The dimensions of the container are selected such that they meet the SMIF Standard. To that end, a hood 1 is shaped such that, given a closed container, a bottom plate 21 (FIG. 2) is arranged mortised into a lower edge region 6 of the hood. Shaped portions 4 provided at opposite sides of the hood 1 serve for the acceptance of locking levers 50 (see FIG. 3 and FIG. 4), whereby recesses 5 for receiving projecting edges 42 are provided. A projecting, upper hood edge 3 that comprises notchings 7 and partly rounded corners 8 and a projecting or salient lower hood edge 2 are designed for the manipulation of the container with robots. 
     The interior of the container is shown in greater detail in FIG. 2 wherein dimensions of the bottom plate 21 are matched such to those of the hood 1 that the bottom plate 21 is mortised in the lower edge region 6 of the hood 1 when the container is closed. Elevations 23 on the upper side of the bottom plate 21 serve the purpose of locking a processing cassette filled with semiconductor wafers (not shown in the figure) residing on the bottom plate 21, and are matched to the dimensions of the processing cassette employed. Given automatic opening of the container, the bottom plate 21 can fulfill the function of a door that, after the locking levers 50 (see FIG. 3 and FIG. 4) have been opened, is withdrawn perpendicularly down from the hood 1 while the hood 1 is held at the salient, lower edge 2. Bottom plate 21 is withdrawn together with the processing cassette residing on it. Various formed portions 24 are provided at the underside of the bottom plate 21, these being designed for machine manipulation when opening and closing the container. The bottom plate 21 comprises an all-around sealing lip 22 that projects over the edge of the bottom plate 21 and is firmly joined thereto, this sealing lip 22 being composed of the same material of which the bottom plate 21 is manufactured. The shape of the sealing lip 22 and of a sealing surface 9 of the hood 1 are designed such that surface pressure is constant over the area of the sealing lip when the container is closed. Optimum tightness is thereby achieved, as is a long useful life of the sealing lip as a result of low wear. 
     A semiconductor wafer securing means 31 is provided for semiconductor wafers (not shown in the figure) arranged vertically above one another in a processing cassette. 
     The semiconductor wafer securing means 31 is interchangeably attached to an inside surface of the hood 1 on the basis of two nub-depression joints 32, 33. The semiconductor securing means 31 comprises a securing plate 37 movably suspended by film hinges 34, 35, 36; this securing plate being brought into position by a guide plate 38 residing in contact with the bottom plate 21 when the container is closed. The movement of the semiconductor wafer securing means 31 when the container is opened and closed ensues without the occurrence of sliding friction that could release particles that contaminate the semiconductor wafers. The dimensions of the semiconductor wafer securing means 31 and the positioning of the processing cassette are selected such that the semiconductor wafer securing means 31 does not touch the semiconductor wafers in the closed condition of the container. The distance between the securing plate 37 of the semiconductor wafer securing means 31 and the semiconductor wafers given a closed container can, for example, be 0.1 mm through 1 mm and can, for example, be approximately 3 mm during loading and unloading. The semiconductor wafer securing means 31 should not touch the semiconductor wafers in the normal case but should hold their ability to move laterally within limits given the occurrence of increased, horizontal accelerations. 
     Since the materials of which the container components are manufactured should release optimally few contaminating particles, it is expedient to manufacture all container components of plastics that are free of exhalations. In order to avoid static charges that could damage electron components and circuits, it is advantageous to manufacture the inside region of the container of anti-static material. Plastics that meet these demands are carbon-laced polypropylene and polycarbonate. A further advantage in employing plastic materials is the possibility of manufacturing colored or transparent parts. For example, the hood 1 of the container can be manufactured of transparent, ultraviolet-impermeable plastic, whereby opening of the container for inspecting its contents that involves a risk of producing contaminations becomes superfluous. By employing only plastic materials, metal contaminations of the semiconductor wafers are avoided. The resistance to abrasion of plastic material is also important, particularly for the semiconductor wafer securing means. For example, polyethylene is extremely well-suited for manufacturing the semiconductor wafer securing means that comprises thin material strips as film hinges. 
     In FIGS. 3 and 4, the locking lever 50 is shown which comprises two projecting edges 42 that engage into the recesses 5 of the hood 1 when the container is closed. Projecting edges 43 that engage into corresponding recesses of the bottom plate 21 are provided at the opposite side of the locking lever. Both lever arm 41 and lever link 44 are resilient elements 
     FIG. 5 illustrates the semiconductor wafer securing means 31 which comprises a fastening plate 39 that can be interchangeably secured to two positions on the inside surface of the hood 1 of the container by means of nubs 32. The securing plate 37 is movably attached to the fastening plate 39 via two articulated plates 40 each of which comprises two film hinges 34, 35. The guide plate 38 that is secured to the securing plate 37 by a film hinge 36 is shaped such that it converts the vertical motion of the bottom plate 21 that occurs when the container is closed into a horizontal motion of the securing plate 37. As a result of this novel design of the semiconductor wafer securing means in the container, no additional particles are released when semiconductor wafers are loaded into the container or unloaded from the container. The film hinges comprise a thickness of less than 0.2 mm for optimum functionability. 
     In FIG. 6, the sealing lip 22 is shown rigidly joined to the bottom plate 21. Universally known plastic processing methods make it possible to manufacture the bottom plate 21 together with the sealing lip 22 of one piece, for example of polypropylene or of polycarbonate. The sealing lip 22 is shaped such that the surface pressure over the area of the sealing lip is constant when the container is closed. This can be achieved in that a thickness 63 of the sealing lip 22 measured in the direction of pressing power tapers parabolically in an outward direction, whereby an upper, inside surface 62 and a lower, outside surface 61 of the sealing lip 22 have different curvatures. Since the sealing surface 9 of the hood 1 drops at an angle of about 10° from the horizontal toward the interior 65 of the container and the sealing lip 22 projects beyond the upper edge of the bottom plate 21 with an angle greater than 10°, the sealing lip 22 cannot snap over when the sealing lip 22 is pressed against the sealing surface 9. 
     FIG. 7 illustrates alternative embodiments of the container wherein a bottom plate 21A is shaped broadened under the projecting sealing lip 22, so that a space 64A formed under the sealing lip 22 is reduced in size in comparison to a space 64 under the sealing lip as shown in the embodiment in FIG. 6. 
     It is provided in a further development of the invention, as shown in FIG. 8, that an elastically deformable sealing ring 72 be arranged as a seal element between the sealing surface 9 of the hood 1 and a bottom plate 21B, this sealing ring 72 being covered with a strip of flexible, low-abrasion film 71 of, for example, plastic. The plastic film 71 thereby protects the interior 65 of the container against contamination due to the substances emitted by the sealing ring 72. The sealing ring 72 can thereby be guided in a channel 74 provided in the bottom plate 21B. The plastic film 71 can be secured to the surface and the outside edge of the bottom plate 21 by bonded plastic connections 73. Further advantages are offered by an arrangement wherein the plastic film 71 forms a hermetically closed, air-filled cavity 75 with the bottom plate 21B and the sealing ring 72 and is not tensed in either the open or closed position of the container. When the container is closed, such an arrangement experiences an elevation of the pressure in the cavity 75, this, given pressure equalization with the interior 65 of the container via the flexible plastic film 71, increasing the interior pressure of the container. The interior pressure of the container can thereby be increased by about 10 mbar. 
     FIG. 9 illustrates a closed container wherein the sealing ring 72 is compressed and the plastic film 71 is partly inverted into the interior 65 of the container due to the over-pressure in the cavity 75. A pressure equalization between the cavity 75 and the interior 65 of the container is produced in this condition. The sealing ring can be an expanded plastic part. The compression properties are optimum given a Shore hardness of about 5°. A polyethylene film having a thickness of about 0.2 mm excellently meets the demands made of the film.